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Static Pressure Control Static Pressure Control LoopLoop
The purpose of the static The purpose of the static pressure control loop is to pressure control loop is to maintain an optimal static maintain an optimal static pressure in the ductwork.pressure in the ductwork.The control loop senses the static The control loop senses the static
pressure in the ductwork and compares pressure in the ductwork and compares it to a setpoint. The controller sends an it to a setpoint. The controller sends an output signal to the VSD which controls output signal to the VSD which controls the fan speed. Adjusting the fan speed the fan speed. Adjusting the fan speed
impacts the static pressure in the impacts the static pressure in the ductwork.ductwork.
Static Pressure Control Static Pressure Control LoopLoop
Duct Static TemperatureDuct Static Temperature
DDC Control CalculationsDDC Control Calculations
Signal to VSD ControllerSignal to VSD Controller
Fan SpeedFan Speed
Air flow through ductworkAir flow through ductwork
Static Pressure Control Static Pressure Control LoopLoop
Air FlowAir FlowAir FlowAir Flow
DDC ControllerDDC Controller Tuning ParametersTuning Parameters
Cooling CoilCooling Coil
Static PressureStatic PressureSensorSensor
VariableVariableSpeed DriveSpeed Drive
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop Setpoint
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointSelecting the correct setpoint is very Selecting the correct setpoint is very
important for system performance.important for system performance.
The optimal setpoint cannot be The optimal setpoint cannot be calculated in advance. It must be calculated in advance. It must be
determined in the field. The current determined in the field. The current practice of fixing the setpoint wastes practice of fixing the setpoint wastes
a lot of energy.a lot of energy.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointIf the setpoint is too low, some boxes If the setpoint is too low, some boxes
will not be able to get enough air to will not be able to get enough air to provide comfort.provide comfort.
Some boxes will have their damper Some boxes will have their damper 100% open but the actual flow will 100% open but the actual flow will still be less than the desired flow. still be less than the desired flow. These boxes are “starved” and the These boxes are “starved” and the VAV box will not be able to achieve VAV box will not be able to achieve the space temperature setpoint. the space temperature setpoint.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointIf the setpoint is too high, fan energy If the setpoint is too high, fan energy
is wasted.is wasted.
The higher the static pressure The higher the static pressure setpoint, the harder the fan must setpoint, the harder the fan must
work to maintain the setpoint.work to maintain the setpoint.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointIf the setpoint is too high, the noise of If the setpoint is too high, the noise of
the system will increase.the system will increase.
If the static pressure setpoint is If the static pressure setpoint is high, all of the VAV box dampers high, all of the VAV box dampers
will only need to open a little bit to will only need to open a little bit to achieve the desired airflow. Airflow achieve the desired airflow. Airflow
going through a small opening going through a small opening generates a lot of noise.generates a lot of noise.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointIf the setpoint is too high, VAV box control If the setpoint is too high, VAV box control
may go unstable.may go unstable.
With a high static pressure setpoint, the With a high static pressure setpoint, the throttling range of the damper to go throttling range of the damper to go
from minimum flow to maximum flow is from minimum flow to maximum flow is decreased. A small control adjustment decreased. A small control adjustment to damper position has a large effect to damper position has a large effect
on airflow. The flow control loop is very on airflow. The flow control loop is very sensitive and may cycle, depending on sensitive and may cycle, depending on
the control loop parameters.the control loop parameters.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointDuring air balancing, use a high During air balancing, use a high
setpoint to make sure that none of setpoint to make sure that none of the boxes are starved.the boxes are starved.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointChoosing the optimal setpoint:Choosing the optimal setpoint:
1.1. Enter a low setpoint so that Enter a low setpoint so that some of the some of the VAV boxes are VAV boxes are starved with damper starved with damper 100% 100% openopen
2.2. Increase the static pressure Increase the static pressure setpoint setpoint until all of the VAV until all of the VAV boxes are in control; boxes are in control; damper less damper less than 100% openthan 100% open
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointStatic Pressure Reset Energy Saving Strategy:Static Pressure Reset Energy Saving Strategy:
1.1. Every five minutes, check the position Every five minutes, check the position of of all VAV box dampers.all VAV box dampers.
2.2. If the damper which is open the most If the damper which is open the most is is more than 90% open, increase the more than 90% open, increase the
static pressure setpoint by 50 Pascal.static pressure setpoint by 50 Pascal.
3.3. If the damper which is open the most If the damper which is open the most is is less than 80% open, decrease the less than 80% open, decrease the static static pressure setpoint by 50 Pascal.pressure setpoint by 50 Pascal.
Static Pressure Control Static Pressure Control LoopLoop
Zone AZone A““Hot”Hot”
Zone BZone B““Warm”Warm”
Zone CZone C““Cool”Cool”
Static Pressure Setpoint = 100 PascalStatic Pressure Setpoint = 100 PascalFan Fan Speed Speed of 40%of 40%
FlowFlowDD
400400DamperDamper
100%100%FlowFlowAA
300300
FlowFlowDD
250250DamperDamper
70%70%FlowFlowAA
250250
FlowFlowDD
150150DamperDamper
30%30%FlowFlowAA
150150
FlowFlowDD = Desired Flow Flow = Desired Flow FlowAA = Actual Flow = Actual Flow
Control Loop SetpointControl Loop Setpoint
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointLow SetpointLow Setpoint
Fan runs at very low speed and uses Fan runs at very low speed and uses little energylittle energy
Zone “A” box is starved, even fully Zone “A” box is starved, even fully open the box cannot supply the open the box cannot supply the
desired air flowdesired air flow
Zone “A” will get hotter because not Zone “A” will get hotter because not enough cool air is being supplied enough cool air is being supplied
Static Pressure Control Static Pressure Control LoopLoop
Zone AZone A““Hot”Hot”
Zone BZone B““Warm”Warm”
Zone CZone C““Cool”Cool”
Static Pressure Setpoint = 200 PascalStatic Pressure Setpoint = 200 PascalFan Fan Speed Speed of 60%of 60%
FlowFlowDD
400400DamperDamper
95%95%FlowFlowAA
400400
FlowFlowDD
250250DamperDamper
50%50%FlowFlowAA
250250
FlowFlowDD
150150DamperDamper
20%20%FlowFlowAA
150150
Control Loop SetpointControl Loop Setpoint
FlowFlowDD = Desired Flow Flow = Desired Flow FlowAA = Actual Flow = Actual Flow
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointCorrect SetpointCorrect Setpoint
Fan runs at low speed and uses Fan runs at low speed and uses some energysome energy
Zone “A” box is almost fully open Zone “A” box is almost fully open but able to supply the desired air but able to supply the desired air
flowflow
All zones will be comfortable as All zones will be comfortable as enough cool air is being supplied enough cool air is being supplied
Static Pressure Control Static Pressure Control LoopLoop
Zone AZone A““Hot”Hot”
Zone BZone B““Warm”Warm”
Zone CZone C““Cool”Cool”
Static Pressure Setpoint = 300 PascalStatic Pressure Setpoint = 300 PascalFan Fan Speed Speed of 80%of 80%
FlowFlowDD
400400DamperDamper
65%65%FlowFlowAA
400400
FlowFlowDD
250250DamperDamper
30%30%FlowFlowAA
250250
FlowFlowDD
150150DamperDamper
10%10%FlowFlowAA
150150
Control Loop SetpointControl Loop Setpoint
FlowFlowDD = Desired Flow Flow = Desired Flow FlowAA = Actual Flow = Actual Flow
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointHigh SetpointHigh Setpoint
Fan runs at medium speed and Fan runs at medium speed and wastes some energy (still better wastes some energy (still better
than CAV)than CAV)
All zones will be comfortable as All zones will be comfortable as enough cool air is being supplied enough cool air is being supplied
Zone “C” will be noisy and it will be Zone “C” will be noisy and it will be difficult to maintain stable controldifficult to maintain stable control
Static Pressure Control Static Pressure Control LoopLoop
Damper PositionDamper Position0%0% 100%100%
AirAirFlowFlow
Control Loop SetpointControl Loop Setpoint
Air flow through VAV box increasesAir flow through VAV box increasesquickly as damper starts to openquickly as damper starts to open
Static Pressure Control Static Pressure Control LoopLoop
Damper PositionDamper Position0%0% 100%100%
AirAirFlowFlow
200 Pascal200 Pascal
500 Pascal500 Pascal
100 Pascal100 Pascal
Static PressureStatic Pressure
Control Loop SetpointControl Loop Setpoint
Characteristic curve of VAV box changesCharacteristic curve of VAV box changeswith the system static pressure.with the system static pressure.
Static Pressure Control Static Pressure Control LoopLoop
Damper PositionDamper Position0%0% 100%100%
AirAirFlowFlow
200 Pascal200 Pascal
500 Pascal500 Pascal
100 Pascal100 Pascal
Static PressureStatic Pressure
Minimum FlowMinimum Flow
Maximum FlowMaximum Flow
VAV box controller modulates airflowVAV box controller modulates airflowbetween minimum and maximum airflow.between minimum and maximum airflow.
Control Loop SetpointControl Loop Setpoint
Static Pressure Control Static Pressure Control LoopLoop
0%0% 100%100%
AirAirFlowFlow
200 Pascal200 Pascal
500 Pascal500 Pascal
100 Pascal100 Pascal
Static PressureStatic Pressure
Minimum FlowMinimum Flow
Maximum FlowMaximum Flow
50%50%
With system static pressure of 200 Pascal,With system static pressure of 200 Pascal,damper is modulated 2% to 50%.damper is modulated 2% to 50%.
Control Loop SetpointControl Loop Setpoint
Static Pressure Control Static Pressure Control LoopLoop
0%0% 100%100%
AirAirFlowFlow
200 Pascal200 Pascal
500 Pascal500 Pascal
100 Pascal100 Pascal
Static PressureStatic Pressure
Minimum FlowMinimum Flow
Maximum FlowMaximum Flow
15%15%
With system static pressure of 500 Pascal,With system static pressure of 500 Pascal,damper is modulated 1% to 15%.damper is modulated 1% to 15%.
Control Loop SetpointControl Loop Setpoint
Static Pressure Control Static Pressure Control LoopLoop
0%0% 100%100%
AirAirFlowFlow
200 Pascal200 Pascal
500 Pascal500 Pascal
100 Pascal100 Pascal
Static PressureStatic Pressure
Minimum FlowMinimum Flow
Maximum FlowMaximum Flow
With system static pressure of 100 Pascal,With system static pressure of 100 Pascal,VAV box can never achieve maximum flow.VAV box can never achieve maximum flow.
Control Loop SetpointControl Loop Setpoint
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointIn this example, the VAV box flow In this example, the VAV box flow
control loop is three times as control loop is three times as sensitive when the static pressure is sensitive when the static pressure is 500 Pascal as compared to when the 500 Pascal as compared to when the
static pressure is 200 Pascal. An static pressure is 200 Pascal. An overly sensitive control loop will overly sensitive control loop will
cycle.cycle.
Static Pressure Control Static Pressure Control LoopLoop
Control Loop SetpointControl Loop SetpointDoes dynamically resetting the static Does dynamically resetting the static pressure setpoint really save energy as pressure setpoint really save energy as compared to having a fixed setpoint?compared to having a fixed setpoint?
FanFan SizeSize OperationsOperations SavingsSavingsAHU-1AHU-1 125 hp125 hp 24 hours 24 hours 42% 42%AHU-2AHU-2 100 hp100 hp 24 hours 24 hours 37% 37%AHU-3AHU-3 50 hp50 hp OccupiedOccupied 19% 19%AHU-4AHU-4 25 hp25 hp OccupiedOccupied 20% 20%
Source : ASHRAE Journal April 1993Source : ASHRAE Journal April 1993
Static Pressure Control Static Pressure Control LoopLoop
Variable Speed DriveVariable Speed Drive
Static Pressure Control Static Pressure Control LoopLoop
Variable Speed DriveVariable Speed DriveThe Variable Speed Drive (VSD) The Variable Speed Drive (VSD)
Controller is typically provided with Controller is typically provided with the fan and accepts a setpoint signal the fan and accepts a setpoint signal
(0-10VDC or 4-20mA).(0-10VDC or 4-20mA).
The VSD provides voltage free The VSD provides voltage free contacts indicating a tripped motor.contacts indicating a tripped motor.
Static Pressure Control Static Pressure Control LoopLoop
Variable Speed DriveVariable Speed DriveVoltageVoltage
FrequencyFrequency
100%100%
NominalNominalFrequencyFrequency
Variable Voltage Variable FrequencyVariable Voltage Variable Frequency
Based on the control signalBased on the control signalinput, the VSD will adjustinput, the VSD will adjustthe voltage and frequencythe voltage and frequency
to the fan motor.to the fan motor.
VSD reacts to changes inVSD reacts to changes insetpoint by changing thesetpoint by changing thevoltage and frequency.voltage and frequency.
Changes are applied slowlyChanges are applied slowlybased on acceleration andbased on acceleration anddeceleration parameters.deceleration parameters.
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure Sensor
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorThe conventional approach requires The conventional approach requires
the static pressure sensor to be the static pressure sensor to be located at the point of minimum located at the point of minimum static pressure in the ductwork.static pressure in the ductwork.
For one long run of ductwork, this For one long run of ductwork, this pointpoint
is 1/2 to 2/3 of the way along.is 1/2 to 2/3 of the way along.
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorConventional ApproachConventional Approach
StaticStaticPoint of Lowest
Pressure inSystem
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorIf the ductwork has two branches, the If the ductwork has two branches, the
static pressure in each branch is static pressure in each branch is taken and the lower of the values is taken and the lower of the values is
used to control the fan.used to control the fan.
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorConventional ApproachConventional Approach
Low Signal SelectLow Signal Select
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorFor example, if branch “A” serves the For example, if branch “A” serves the
east side of the building and branch east side of the building and branch “B”serves the west side of the “B”serves the west side of the
building then in the morning, the fan building then in the morning, the fan would be controlled by sensor “A” would be controlled by sensor “A” and in the afternoon, the fan would and in the afternoon, the fan would
be controlled by sensor “B”.be controlled by sensor “B”.
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorIf the static pressure setpoint is If the static pressure setpoint is
continuously reset based on VAV damper continuously reset based on VAV damper position:position:
Energy will be saved as fan only works asEnergy will be saved as fan only works as hard as is necessary hard as is necessary
Placement of static pressure sensor doesPlacement of static pressure sensor does not matter - as long as airflow is not not matter - as long as airflow is not turbulent : away from fan, 5 duct turbulent : away from fan, 5 duct diametersdiameters away from bends, dampers, reducers, etc. away from bends, dampers, reducers, etc.
Static Pressure Control Static Pressure Control LoopLoop
Converts differential Converts differential pressure betweenpressure between
H and L into a H and L into a signal.signal.
When H is inside When H is inside duct and L is duct and L is
outside duct static outside duct static pressure of duct is pressure of duct is
measured.measured.
Static Pressure SensorStatic Pressure Sensor
Static Pressure Control Static Pressure Control LoopLoop
Static Pressure SensorStatic Pressure SensorCalibrate a static pressure sensor by:Calibrate a static pressure sensor by:
Disconnect static pressure sensor.Disconnect static pressure sensor. Inject a 2.00 VDC voltage and verify Inject a 2.00 VDC voltage and verify
thatthat the DDC Controller reads zero. the DDC Controller reads zero.
Inject a 10.00 VDC voltage and Inject a 10.00 VDC voltage and verify thatverify that the DDC Controller reads maximum. the DDC Controller reads maximum.
Replace static pressure sensor.Replace static pressure sensor.
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